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Merge pull request #294 from chriseth/newfaq

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Overflow check.
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chriseth 2015-12-10 15:47:09 +01:00
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docs/frequently-asked-questions.rst
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@ -742,6 +742,20 @@ If you want to send 20 Ether from a contract to the address `x`, you use `x.send
Here, `x` can be a plain address or a contract. If the contract already explicitly defines Here, `x` can be a plain address or a contract. If the contract already explicitly defines
a function `send` (and thus overwrites the special function), you can use `address(x).send(20 ether);`. a function `send` (and thus overwrites the special function), you can use `address(x).send(20 ether);`.
What does the following strange check do in the Custom Token contract?
======================================================================
::
if (balanceOf[_to] + _value < balanceOf[_to]) throw;
Integers in Solidity (and most other machine-related programming languages) are restricted to a certain range.
For `uint256`, this is `0` up to `2**256 - 1`. If the result of some operation on those numbers
does not fit inside this range, it is truncated. These truncations can have
`serious consequences <https://en.bitcoin.it/wiki/Value_overflow_incident>`_, so code like the one
above is necessary to avoid certain attacks.
More Questions? More Questions?
=============== ===============